Filter element

ABSTRACT

A filter element for removing material that is entrained in a gas stream comprises a wall of a filtration medium which defines a hollow space, for a gas stream to flow from the space through the wall. The filter element includes an end cap having an inlet tube for a gas stream to be supplied to the space. The tube has at least one helically extending rifle formation in its internal wall by which a helical flow is imparted to the gas stream when it leaves the tube.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is continuation of copending International ApplicationNo. PCT/GB05/002928 filed Jul. 26, 2005, which designated the UnitedStates, the disclosure of which is incorporated herein by reference, andwhich claims priority to Great Britain Patent Application 0417457.9,filed Aug. 5, 2004.

BACKGROUND OF THE INVENTION

This invention relates to a filter element for removing material that isentrained in a gas stream.

SUMMARY OF THE INVENTION

Filtration of gas in a compressed gas stream is generally required sothat the gas is sufficiently clean for a subsequent application, or tominimise adverse effects of impurities on components of the system. Forexample, removal of compressor oil can be required to minimise chemicalcontamination and accumulation on valves which might lead to malfunctionof the valves, and removal of particulate solid material can be requiredto minimise abrasion.

Coalescing filters are used to collect liquid that is entrained in a gasstream by causing aerosol droplets of the liquid to coalesce and collectas drops, which can flow as a liquid. They generally comprise severallayers of filter media. For example, filter elements generally comprisea cylindrical filtration layer and a cylindrical anti-reentrainmentbarrier or a “drainage layer” surrounding the filtration layer on theoutside of the filter element. The density and thickness of the medialayers are selected according to the flow rate of the gas stream, thelevel and nature of the impurities in the gas stream, the level ofimpurity that is sought in the gas stream after filtration and so on.

Common filter constructions comprise a tubular filter element mounted ina tubular housing. The gas to be filtered passes radially through thewall of the filter element. Solid particles entering the filter housingare collected by the filter element. Liquid droplets, possibly asaerosols, entrained in the gas are collected by the filter element. Thedroplets can coalesce to form drops, which then collect at the base ofthe filter element for drainage. Clean gas can then be discharged fromthe filter.

Coalescing filter elements of this type can be arranged so that gas tobe filtered flows radially inwardly through the filter media whichprovide the wall of the element. The gas is supplied to the cavityaround the element, between the element and the wall of the housing. Itthen passes inwardly through the element for discharge from the spacewithin the element to the end use application.

It is more common for coalescing filter elements to be arranged so thatgas to be filtered flows radially outwardly through the element wall:the gas is then supplied to the cavity within the element and passesoutwardly through the element wall for discharge from the space betweenthe outer surface of the element and the wall of the housing. Elementsof this latter kind are sometimes referred to as “in-to-out” filterelements, reflecting the direction of flow of gas through the filtermedium.

It is preferable that contaminant material which is entrained to flow inthe gas stream should be distributed evenly through the length of thefilter element. PCT Application No. WO-A-2004/09210 discloses a filterelement having an inlet tube which extends from an end cap into thehollow space defined by the element wall. The inlet tube can help toensure that the gas stream, containing contaminant material, is directedto areas of the element wall which are spaced from the inlet end of theelement. The disclosed filter element can have peripheral openings inits end cap for supply of the gas stream to areas of the element wallclose to the inlet end. These openings can be defined by vanes which canbe configured to impart a helical glow to gas flowing through theopenings. A vane can also be provided in the inlet tube.

The present invention provides a filter element having an inlet tube inits end cap, which has at least one helically extending rifle formationin its internal wall by which a helical flow is imparted to the gasstream when it leaves the tube.

Accordingly, in one aspect, the invention provides a filter element forremoving material that is entrained in a gas stream, the filter elementcomprising:

a wall of a filtration medium which defines a hollow space, for a gasstream to flow from the space through the wall, and

an end cap having an inlet tube for a gas stream to be supplied to thespace, the tube having at least one helically extending rifle formationin its internal wall by which a helical flow is imparted to the gasstream when it leaves the tube.

The element of the invention has the advantage that helical flow can beimparted to the gas stream by virtue of the rifle formation in theinternal wall of the tube for the gas stream to tend to flow outwardlywhen it leaves the tube, so that it flows towards the internal wall ofthe filter element that is provided by the filtration medium. It appearsthat the helical flow that is imparted to gas entering the filterelement can lead to a more even distribution of contaminant material inthe gas stream over the length of the filter element: particles anddroplets of contaminant material will generally tend to be relativelyheavy, and the helical flow can result in them being directed furtherinto the filter element than has been the case in known elements.Furthermore, primary separation of liquid droplets from the gas streamcan be facilitated as a result of the helical flow of gas entering theelement. This can facilitate collection of aerosol liquid droplets toform drops on the surface of the coalescing filter media, so that thedroplets then collect within the media and flow to the base of theelement. These benefits in terms of improved filtration efficiency aresignificant. Furthermore, it has been found that the operating lifetimeof a filter element is not affected adversely by localised collection ofentrained liquid droplets on the wall of the filter element close to theinlet port.

It has been found surprisingly that the helical flow that arises fromthe rifle formation gives rise to the sufficient outward flow of the gasstream when it leaves the tube, while giving rise to less resistance toflow of gas, and to reduced energy losses arising from the pressure dropacross the element. This combination of properties can therefore giverise to reduced operating costs compared with known filter elementconstructions such as that disclosed in PCT Application No.WO-A-2004/09210.

The nature of a helical flow which is imparted to gas entering thehollow space within the tubular element can be characterised as atwisted flow. It can also be characterised as a cyclonic gas flow. Acharacteristic of the flow that is imparted to the gas is that, insteadof flowing in a direction which is parallel to the element axis, themovement of the gas includes a component which involves the gas flowingaround the said axis.

A further advantage of imparting a helical flow to gas entering thehollow space within the tubular element is that the reaction of theelement to the helical flow imparted by the vanes can be relied on tominimise the risk of the filter element becoming detached from itshousing. This advantage applies in particular for example when thefilter element is fitted into its relevant housing part by a twistingaction, for example relying on threaded or bayonet formations.

Preferably, the tube extends into the hollow space that is defined bythe wall of the filtration medium. Preferably, the tube is approximatelystraight, at least in the portion of its length that is within thehollow space. Preferably the ratio of the length of the tube (measuredfrom the inner surface of the end cap on which the tube is mounted) tothe length of the filter element (measured between the inner surface ofthe opposite end caps) is at least about 0.15, more preferably at leastabout 0.20, especially at least about 0.25.

The length of the inlet tube is measured from the point on the externalsurface of the end cap which closes the hollow space where the inlettube is connected to the end cap, to the end of the tube which is distalfrom the end cap.

Preferably, the inlet tube is co-axial with the end cap. This can aidthe even distribution of the contaminated gas across the wall of thefilter element. However, it will be appreciated that the inlet tube neednot be co-axial with the end cap.

The inlet tube can be formed integrally with the end cap, or can beformed separately and subsequently fastened to the end cap.

It can be advantageous to form the inlet tube separately as it allowsthe for the production of a variety of different inlet tubes that can befastened to standard end caps. However, it can also be advantageous toform the end cap and the inlet tube as one piece as this can reducemanufacturing costs. This can especially be the case if a variety ofdifferent inlet tubes are not used in different applications.

If the inlet tube is formed separately from the end cap, then theinterface between the end cap and the inlet tube should form a fluidtight seal. Preferably, the tube and the end cap are formed form thesame material. Preferably, the inlet tube can be fastened to the firstend cap so that it can be subsequently removed. For example, preferablythe inlet tube is fastened to the end cap through the use of amechanical fastening such as a latch, co-operating screw threads, orengaging bayonet formations. More preferably, the inlet tube and thefirst end cap are shaped and sized so that the inlet tube is held withinthe end cap by the friction forces between the inlet tube and the endcap.

It can be advantageous in some applications to fasten the inlet tube tothe end cap so that the inlet tube cannot be subsequently removed fromthe end cap. In this case, preferably the inlet tube is fastened to theend cap without the use of a third party material. For example,preferably, the inlet port is fastened to the end cap through the use ofa welding technique, for example, ultrasonic or heat welding. However,it will be appreciated that the inlet port can be fastened to the endcap through the use of a third party material such a bonding agent, forexample an adhesive.

Preferably, the end cap has an opening extending around the periphery ofthe inlet tube for supply of the gas stream to the element wall close tothe end cap. This can be advantageous as it can aid the evendistribution of contaminant material across the element wall.

The inlet tube can have at least one opening in its side wall.Preferably, the inlet tube has an upstream band, a middlestream band anda downstream band, the said bands being axially adjacent to one anotherand non-overlapping, having equal axial lengths of at least 5% of thelength of the inlet tube, and being arranged such their planes areperpendicular to the axis of the inlet tube, and in which the proportionof the area of the side wall that is open in the upstream band issmaller than that in the middlestream band, and the proportion of thearea of the side wall that is open in the middlestream band is smallerthan that in the downstream band. This can help ensure that the supplyof gas into the filter element is graded, which can lead to a more evendistribution of gas flowing through the element wall.

Preferably, the opening in the inside wall of the inlet tube is a slitthat has an open end at the downstream open end of the inlet tube andextends towards the upstream open end of the inlet tube. Preferably, theslit extends parallel to the axis of the inlet tube. However, the slitcan extend at an angle to the axis of the inlet tube. For example, theslit can be a helically extending slit. It has been found that ahelically extended slit can provide a more even distribution ofcontaminated material around the element wall. Preferably, the helixangle defined by the slit is at least about 5°, or preferably at leastabout 10°, especially at least about 15°, for example at least about20°. Preferably, the helix angle defined by the formation is not morethan about 75°, but preferably not more than about 60°, for example, notmore than about 50°. The helix angle is the angle between a straightline projecting substantially along a portion of the slit (when viewedfrom the side) and the plane which is perpendicular to the axis of thetube.

Preferably, the helically extending slit extends around the tube axisthrough at least about 360° between its ends, more preferably, at leastabout 720°, for example at least about 1080°. However, it will beappreciated that the helically extending slit need not extend through atleast about 360° between its ends. For example, the helically extendingslit can extend through at most about 180° between its ends.

Preferably, the width of the helically extending slit perpendicular tothe direction of the slit increases towards the downstream open end.However, the helically extending slit need not be wider towards itsdownstream end. For example, the helically extending slit can bearranged so that the helix angle defined by the slit towards itsdownstream end is shallower than towards its upstream end, therebyproviding a tighter helix towards its downstream end, and therefore theproportion of the side wall that is open increases towards thedownstream end.

Preferably, the number of slits in the side wall of the inlet tube ismore than one. Preferably, the number of slits provided in the side wallof the inner tube is between two and ten, more preferably, between fourand eight, especially preferably, between five and seven, for example,six.

When there is more than one slit, preferably, the slits are spacedequally around the inlet tube. For example, when there are two slits,preferably, the slits are spaced apart from each other by about 180°around the inlet tube. When there are four slits, preferably, the slitsare spaced apart from each other by about 90° around the inlet tube.This can help ensure that a gas stream entering the filter element isdistributed evenly around the wall of the filter element.

Preferably, when there is more than one slit in the side wall of theinlet tube, the upstream most points of the slits define a planeperpendicular to the axis of the inlet tube and the width of at leastone slit increases as you travel from its upstream most point to itsdownstream open end. However, this need not be the case. For example,the upstream most points of the slits can be staggered along the lengthof the inlet tube so the slits do not all extend the same axial distancefrom the downstream open end to the upstream open end of the inlet tube.

Preferably, when more than one slit is provided in the side wall of theinlet tube, the shape and configuration of the slits are identical.However, this need not be the case. For example, the width of a firstslit may be wider at its downstream most point than that of a secondslit. Further, a first slit may extend generally helically around theinlet tube, and a second slit may extend generally parallel to the axisof the inlet tube.

When more than one slit is provided in the side wall of the inlet tube,the width of the slits need not increase towards the downstream end. Forexample, the slits may extend towards the upstream open end of the inlettube for different axial lengths so that the number of slits present inthe upstream band is less than the number present in the middlestreamband, and the number of slits present in the middle stream band is lessthan the number present in the downstream band.

The at least one opening in the side wall can be an aperture thatextends generally transversely through the side wall so that it issurrounded by the side wall. When there is only one aperture the size ofthe aperture will increase towards the downstream end of the tube.

Preferably, there are a plurality of apertures. For example, preferablythe number of apertures is at least 2, more preferably at least 10, forexample at least 20 or 25. There can be less than about 75 apertures,preferably less than about 50 apertures, more preferably less than about40 apertures, for example less than about 25 apertures.

Preferably, the apertures are arranged such that the number of apertureslocated in the upstream band is less than the number of apertures in themiddlestream band, and the number of apertures in the middlestream bandis less than the number of apertures in the downstream band. Preferably,the size of the apertures are equal. However, it will be appreciatedthat the size of the apertures can vary. For example, the number ofapertures in the upstream, middlestream and downstream bands can beequal, and the size of the apertures in the upstream band can be smallerthan the size of the apertures in the middle-stream band, and the sizeof the apertures in the middlestream band can be smaller than the sizeof the apertures in the downstream band.

The shape of the apertures can be any regular or irregular shape.Preferably, the apertures are circular, for example, elliptical.However, the apertures can be any shape such as square, hexagonal, andcan be configured so that the apertures extend helically around theinlet tube. Preferably, the shape of the apertures are the same.However, it will be appreciated that the apertures need not be the sameshape as each other. For example, at least one aperture may be circular,and at least another aperture can be hexagonal.

Preferably, the apertures are arranged so that, within each band, theapertures are equally spaced around the inlet tube. For example, ifthere are four apertures present in the upstream band, preferably, theyare spaced apart from each other by about 90° around the inlet tube.Preferably, the apertures are arranged so that, within each band, thepoints at the centres of the apertures define a plane that extendsperpendicular to the axis of the inlet tube.

Preferably, when there is more than one opening, the openings are eitherall slits or all apertures. However, when there is more than oneopening, at least one opening can be a slit and at least one opening canbe an aperture.

Details of a filter element having an inlet tube with at least oneopening in its side wall are disclosed in the co-pending PCT applicationfiled with the present application which claims priority from UK PatentApplication No. 0417462.9. Subject matter that is disclosed in thatapplication is incorporated in the specification of the presentapplication by this reference.

The rifle formation can be provided by at least one ridge which isprovided on the internal wall of the tube. The ridge can be taperedtowards a point at least one of its ends, preferably at each of itsends. This can reduce the resistance to flow of the gas stream over theridge.

The rifle formation can be provided by a plurality of ridges.Preferably, when the rifle formation is provided by more than one ridge,the plurality of ridges are arranged so that adjacent ridges are spacedapart approximately equally around the inlet tube, i.e. the distancebetween each ridge is approximately constant over the length of theridges. Preferably, the number or ridges provided is at least two, morepreferably at least four, for example at least six or at least 10.Preferably, the number of ridges is less than 30, more preferably lessthan 20, for example less than 15.

The rifle formation can be provided by at least one groove which isprovided on the internal wall of the tube. The groove can be defined bytwo ridges which are provided on the internal wall of the tube.

The rifle formation can be provided by a plurality of grooves. Theplurality of grooves are arranged so that adjacent grooves are spacedapart approximately equally around the inlet tube. Preferably, thenumber or grooves provided is at least two, more preferably at leastfour, for example at least six or at least 10. Preferably, the number ofgrooves is less than 30, more preferably less than 20, for example lessthan 15.

Preferably, the helix angle defined by the formation is at least about5°, more preferably at least about 10°, especially at least about 15°,for example at least about 20°. Preferably, the helix angle defined bythe formation is not more than about 75°, more preferably not more thanabout 60°, for example not more than about 50°. The helix angle is theangle between a straight line projecting substantially along a portionof the crest of the formation (when viewed from the side), and the planewhich is perpendicular to the axis of the tube. The helix angle will beselected according to the desired helical flow that is to be imparted tothe gas stream, balanced again the resistance to the flow of the gasstream that arises from the presence in the tube of the formation.

Preferably, the rifling formation extends only part way along the lengthof the tube. Preferably the ratio axial length of the rifling formationto the axial length of the tube is not more than 0.75. More preferably,the ratio axial length of the rifling formation to the axial length ofthe tube is not more than 0.65. Especially preferably, the ratio axiallength of the rifling formation to the axial length of the tube is notmore than 0.5.

Preferably, the rifle formation extends through at least about 360°between its ends, for example at least about 720°. However, it will beappreciated that the rifle formation need not extend through at leastabout 360° between its ends. For example, the rifle formation can extendthrough at most about 180° between its ends.

Preferably, when the rifle formation is provided by more than one grooveor ridge, each groove or ridge extends through at least about 360°between its ends, for example at least about 720°. However, it will beappreciated that each groove or ridge need not extend through at leastabout 360° between its ends. This can be particularly advantageous whena large number of grooves or ridges are provided. For example, eachgroove or ridge can extend through at most 270° between its ends,preferably, at most 180° between its ends.

Preferably, the ratio of the depth of the rifle formation to thetransverse dimension of the tube (which will be its diameter when thecross-section of the tube is circular) is at least about 0.02, morepreferably at least about 0.05. Preferably, the ratio of the depth ofthe rifle formation to the transverse dimension of the tube is not morethan about 0.3, more preferably at least about 0.1.

Preferably, the depth of the rifle formation is at least about 0.5 mm,more preferably at least about 1.0 mm. Preferably, the depth of therifle formation is at most about 5.0 mm, more preferably at most about2.5 mm.

Materials suitable for use in a filtration filter element will beselected according to the nature of the gas that is being filtered, thenature of the contaminants (liquid droplets, aerosols, solid particlesetc) to be filtered from the gas, the pressure differential across thefilter and so on. Such materials are known, including those used byDomnick Hunter Limited in products which are available under the trademark OIL-X. Suitable materials for use as a filtration medium (or afiltration layer) include, borosilicate and other glass fibres,activated carbon minerals, activated silica materials and so on. Afiltration layer can be made from woven fibres. However, as will beappreciated, a filtration layer can be made from sheets of non-wovenfibres. For example, a microfiber filtration layer made from fineorganic or inorganic fibres is preferred. Preferably, a coarser fibrelayer is fitted on the inside of a microfiber filtration layer. Thiscoarser layer can protect a microfiber filtration layer from grosspollution.

Preferably, the filtration layer can comprise a layer of a materialwhich has been folded so that it is fluted (or pleated). This canincrease the surface area of the filtration layer through which airflowing through the filter element will pass. This can also help toincrease the rigidity of the filtration layer.

The filtration layer can be surrounded by an “anti-reentrainment” ordrainage layer on its outside. Drainage layers are especially used infilter elements where the nature of the contaminants to be filtered fromthe gas is in the form of aerosols, or liquid droplets. The drainage offilter element can be any material that is capable of retaining liquidthat has been coalesced by the filtration layer, and is carried to thedrainage layer by a gas stream that flows through the drainage layer.The drainage layer of the filter element will generally be porous, andmade from a material which encourages flow of coalesced liquid towardsthe base of the filter element. Factors affecting the drainagecharacteristics include pore size and structure, and the material of thedrainage layer, including for example the surface energy of liquid whichis in contact with the material. Materials suitable for use in thedrainage layer are used in similar products sold by Domnick HunterLimited under the trade mark OIL-X. Suitable materials includeopen-celled foam plastics, felted fabric material, and expanded foammaterials.

The element can include at least one support for the filtration layer.This can help to retain the filtration layer in its position within thefilter element. This can also increase the rigidity of the filterelement. A support can be provided within the hollow space, positionedagainst the internal surface of the filtration layer. A support can bepositioned outside the filtration layer, for example between thefiltration layer and the drainage layer. Preferably, a first supportmade of a rigid material is positioned within the hollow space againstthe internal surface of the filtration layer, and a second support madeof rigid material is positioned outside the filtration layer.Preferably, the or each support is perforated to allow a gas stream toflow therethrough. The material for the support should have sufficientrigidity to withstand the forces to which the element is exposed, duringassembly of the element and an assembly containing the element, andduring use. The material can be metallic, for example a stainless steel.

Preferably, the filter element has a substantially constantcross-sectional shape along its length. Preferably, the cross-section ofthe filter element is generally round. For example, the filter elementmay be circular or elliptical. However, it will be appreciated that thecross-section of the filter element need not be round. For example, thecross-section could be the shape of a square, triangle, or any otherregular or irregular shape.

The shape of the filter element when viewed along its axis (itscross-section shape) will generally be approximately constant over atleast most of the length of the element. However, it will be appreciatedthat its cross-section shape need not be constant. For example, thefilter element could be conical or pyramidal.

Generally, the filter element will include first and second end caps atopposite ends of the wall of the filtration medium, with the inlet tubeprovided on the first end cap. Generally, the filter element will bearranged in use with the first end cap located above the second end cap,when the first end cap might be referred to as the top end cap, and thesecond end cap referred to as the bottom end cap.

The cross-sections of the end caps and the wall at their interfacesshould be broadly the same so that the end caps can be fitted togetherwith the wall to provide fluid tight seals. Preferably, the filtrationand drainage layers are retained and sealed within a trough which isprovided in the first end cap, so that there is no path for gas to flowpast the filtration medium.

When a second end cap is provided, it can be preferred for thefiltration layer to be retained and sealed within a trough which isprovided in the second end cap, so that there is no path for gas to flowpast the filtration medium. Preferably, the drainage layer extends fromthe wall of the filter element over at least a part of the externalsurface of second end cap, over the face thereof which faces away fromthe first end cap. This can facilitate drainage of coalesced fluid fromthe drainage layer of the filter element, into a reservoir in theassembly from which it can be collected, for example for disposal.Details of a filter element in which the drainage layer extends from thewall of the filter element over at least a part of the external surfaceof second end cap, over the face thereof which faces away from the firstend cap are disclosed in the co-pending PCT application filed with thepresent application which claims priority from UK Patent Application No.0417455.3. Subject matter that is disclosed in that application isincorporated in the specification of the present application by thisreference.

The drainage medium (or one or more layers thereof) can be sealed to anend cap using a quantity of a bonding material such as an adhesive. Thebonding material should be selected according to fluids with which theelement will come into contact when in use so that there are no adversereactions between the bonding material and the fluids. The drainagemedium might be sealed to the first end cap using other techniques suchas welding, for example by localised application of heat, or byultrasonic welding.

Preferably one or each of the end caps is formed from a polymericmaterial. Preferred polymeric materials include polyolefins (especiallypolyethylene and polypropylene), polyesters, polyamides, polycarbonatesand the like. Polymeric materials used for the end caps can bereinforced, for example by fibrous materials (especially glass fibres orcarbon fibres). Materials other than polymeric materials can be used,for example metals. The first and second end caps will generally beformed from the same material or materials.

Preferably one or each of the end caps is formed by moulding, forexample, by injection moulding.

Preferably, the second end cap includes an upstand portion which extendstowards the first end cap, into the space within the filter element.This can help to encourage fluid to flow transversely towards the wallof the element. Preferably, the upstand portion is located centrallywithin the element.

Preferably, the upstand portion of the second end cap tapers inwardlytowards the end thereof which is closer to the first end cap. Theupstand portion can have a substantially constant cross-sectional shapealong its length. Preferably, the cross-section of the upstand portionis generally round, for example elliptical or especially circular.

When the second end cap includes an upstand portion that has a circularcross-section, and when the second end cap includes a flange and a stem,preferably the stem and the upstand portion are co-axial, and preferablythe diameter of the upstand portion is the same as that of the stem.This can ease construction of the second end cap, especially when thesecond end cap is formed by moulding.

In another aspect, the invention provides a filter assembly forcollecting material that is entrained in a gas stream which comprises afilter element as discussed above, and a housing in which the filterelement is received.

The housing can comprise a head and a body. The head and the body can beseparable, providing access to the interior of the housing, especiallyfor replacement of the filter element. The head and body should becapable of being connected to one another to form a fluid tight seal,for example by means of cooperating bayonet formations or by means ofcooperating screw threads.

The housing should provide an inlet port for a gas stream to flow intothe housing, and an outlet port through which gas which has passedthrough the filtration medium can leave the housing. The ports willgenerally be provided in the housing head.

Preferably, the end cap, or when provided, the second end cap (or eachof the end caps), has means for locating the filter element in thehousing of the filter assembly, especially to control the alignment ofthe element in the housing. For example, the end cap can have at leastone rib which is received in an appropriate groove in a housing. Whenthe end cap has the rib, this can also facilitate the loosening of thefilter element from the housing head part when the housing body isrotated relative to the housing head part. Details of a filter assemblyin which the end cap has at least one rib that is received within anappropriate groove in the housing are disclosed in the co-pending PCTapplication filed with the present application which claims priorityfrom UK Patent Application Nos. 0417463.7 and 0428567.2. Subject matterthat is disclosed in that application is incorporated in thespecification of the present application by this reference.

Preferably, the housing includes a reservoir in which coalesced liquidwhich drains from the drainage layer can collect. The reservoir can beprovided by a space within the housing below the filter element.

Preferably, the housing includes a drain outlet for coalesced liquidwhich drains from the drainage layer. The outlet will generally providefor removal of liquid which has collected in a reservoir. The drainshould preferably be capable of opening without depressurising thehousing. A suitable drain mechanism is disclosed in European Patent No.EP-A-81826.

The housing should be formed from a material which is capable ofwithstanding the internal pressures to which it is subjected when inuse. Metals will often be preferred, for example aluminium and alloysthereof, and certain steels.

The inlet at the end cap of the filter element should be capable ofcoupling with a part in a housing for the element so that a tight sealis formed. This can ensure that all gas that enters the inlet of thefilter assembly, enters the filter element. Techniques for sealing inletports are known, for example as disclosed in PCT Application No.WO-A-99/30798.

Preferably, the parts of the housing fit together for the purposes ofconnection in such a way that the body part as a male part fits withinthe head part as a female part.

Preferably, the inlet on the end cap of the filter element has acompressible O-ring seal for forming a fluid tight seal with the headpart of the housing.

A flow conduit having a first conduit opening for communication with aport (generally the inlet port in the housing head) for the gas that isto be filtered and a second conduit opening for communication with theinlet tube of the end cap, can be provided. Generally, the flow of thegas stream towards and away from the assembly will be horizontal. Thefilter element will generally be arranged vertically so that the housinghead is at the top of the housing with the filter element locateddepending below it. In such constructions, the axis of the first conduitopening and the axis of the second conduit opening will not be aligned.Generally, the axis of the first conduit opening will be substantiallyperpendicular to the axis of the second conduit opening.

Preferably, the flow conduit will be configured to provide a smooth flowpath for a gas which flows between the first and second conduitopenings. The smooth flow path can be constructed to reduce restrictionof the flow of the gas stream compared for example with a flow conduitwhich presents a discontinuous flow path which is sharply angled orcontains steps or other obstructions.

The flow conduit can be provided as part of the end cap. However,preferably the flow conduit is formed separately from the end cap andsubsequently fastened to the end cap. This can reduce the costs ofmanufacturing the end cap, especially when the end cap and flow conduitand formed by a moulding process. The interface between the inlet in thefilter element and the flow conduit should form a fluid tight seal toensure that all gas that flows through the flow conduit enters thefilter element. Preferably, the flow conduit and the end cap are formedform the same material. Preferably, the flow conduit can be fastened tothe end cap so that it can be subsequently removed. For example,preferably the flow conduit is fastened to the end cap through the useof a mechanical fastening such as a latch, co-operating screw threads,or engaging bayonet formations. More preferably, the flow conduit andthe end cap are shaped and sized so that the flow conduit is held withinthe end cap by the friction forces between the flow conduit and the endcap.

Preferably, the flow conduit extends into the head part of the housingwhen the assembly is assembled. A seal can be provided in one or both ofthe surfaces of the housing and the flow conduit which contact oneanother. For example, a seal can be provided in a face of the flowconduit around the first conduit opening. The seal can be provided in agroove in that face. It can be provided as a separable component of theflow conduit. It might be formed as an integral part of the flowconduit, for example as a result of being formed by moulding in place.The material for seals in an assembly according to the invention will beselected according to the application for the assembly; the seal willgenerally be provided by an elastomeric material.

The head part can present a bore in which the end of the flow conduit isreceived. Preferably, the head part of the housing has internal wallswhich define a primary chamber within it which communicates with one ofthe ports and also with the hollow space within the tubular filterelement. Generally, the axis of the port and the axis of the tubularfilter element will not be aligned. Preferably, a seal is providedbetween the internal wall of the primary chamber at or towards the freeend thereof and the flow conduit at the second conduit opening thereof.Preferably, the flow conduit extends from the hollow space into theprimary chamber. It has been found the provision of a flow conduit thatextends from the hollow space into the primary chamber can give rise tosignificant advantages, including that any turbulence in the flow of gaswithin the chamber between the port and the hollow space resulting fromthe non-alignment of the axes can be reduced. Further, such an assemblyhas fewer constraints on the design of the head part with a view tominimising flow resistance compared with other assemblies, such as thatdisclosed in PCT Application No. WO-02/38247. Details of a filterassembly having a primary chamber into which the flow conduit extendsare disclosed in the co-pending PCT application filed with the presentapplication which claims priority from UK Patent Application No.0417458.7. Subject matter that is disclosed in that application isincorporated in the specification of the present application by thisreference.

The flow conduit can be made with additional features during itsmanufacture. For example, a port might be formed in it for connection tomeans for indicating the pressure within the conduit. Such ports areknown, for example as disclosed in PCT Application No. WO-A-99/30798.Details of an alternative arrangement for such a port are disclosed inthe co-pending PCT application filed with the present application whichclaims priority from UK Patent Application No. 0417458.7. Subject matterthat is disclosed in that application is incorporated in thespecification of the present application by this reference.

The flow conduit can contain at least one vane positioned within it sothat the flow of gas along the conduit between the first and secondconduit openings passes over the vane and is smoothed by it. Theprovision of a vane in the flow conduit has been found to reduce theresistance to flow of gas through the flow conduit compared with a flowconduit which does not include a vane. This can enable the efficiency ofa filter element to be enhanced compared with known assemblies of thisgeneral kind. Details of a filter assembly having a flow conduit whichhas a vane are disclosed in the co-pending PCT application filed withthe present application which claims priority from UK Patent ApplicationNo. 0417464.5. Subject matter that is disclosed in that application isincorporated in the specification of the present application by thisreference.

Preferably, the flow conduit contains at least two of the said vanes.

The flow conduit can be formed in first and second matable pieces, inwhich the first piece comprises a first part of the flow conduit walland the vane, and the second piece comprises a second part of the flowconduit wall which has a recess formed in it in which the end of thevane that is remote from the first part of the conduit wall can bereceived when the first and second pieces are mated. Preferably, thefirst and second matable pieces of the flow conduit mate in a planewhich contains the first and second conduit openings. Details of afilter assembly in which the flow conduit can be formed in first andsecond matable pieces is disclosed are disclosed in the co-pending PCTapplication filed with the present application which claims priorityfrom UK Patent Application No. 0417464.5. Subject matter that isdisclosed in that application is incorporated in the specification ofthe present application by this reference.

When a second end cap is provided, and when the drainage layer extendsfrom the wall of the filter element over at least a part of the externalsurface of second end cap, over the face thereof which faces away fromthe first end cap, the part of the drainage layer between the second endcap and the housing can be compressed by at least one longitudinallyextending fin. This can facilitate drainage of coalesced fluid from thedrainage layer of the filter element, into a reservoir in the assemblyfrom which it can be collected, for example for disposal. The effectiveengagement of the housing wall with the second end cap, through the finand the drainage layer, can also help to locate the filter element inthe housing transversely. This can facilitate the formation of areliable seal between the housing and the filter element which mightotherwise be disturbed if the element is able to move transverselywithin the housing. The transverse location of the element in thehousing operates in conjunction with the axial location provided by theinterengaging rib and groove. Details of a filter assembly in which thepart of the drainage layer between the second end cap and the housingare compressed by at least one longitudinally extending fin aredisclosed in the co-pending PCT application filed with the presentapplication which claims priority from UK Patent Application No.0417459.5. Subject matter that is disclosed in that application isincorporated in the specification of the present application by thisreference.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, and with reference to the accompanying drawings, in which:

FIG. 1 is sectional side elevation through a filter assembly accordingto the present invention which comprises a filter element and thehousing in which the element is located when in use.

FIG. 2 is a sectional side elevation through a filter element accordingto the present invention.

FIG. 3 is a sectional side elevation of the top end cap of the filterelement shown in FIG. 2.

FIG. 4 is a schematic perspective view of the top end cap of the filterelement shown in FIG. 2.

FIG. 5 is a schematic top view of the top end cap of the filter elementshown in FIG. 2.

FIG. 6 is a schematic sectional elevation of the top end cap of thefilter element shown in FIG. 2 illustrating a second embodiment of theinlet tube of the filter element.

FIG. 7 is a schematic sectional elevation of the top end cap of thefilter element shown in FIG. 2 illustrating a third embodiment of theinlet tube of the filter element.

FIG. 8 is a schematic sectional elevation of the top end cap of thefilter element shown in FIG. 2 illustrating a fourth embodiment of theinlet tube of the filter element.

FIG. 9 is a schematic sectional elevation of the top end cap of thefilter element shown in FIG. 2 illustrating a fifth embodiment of theinlet tube of the filter element.

FIG. 10 is a schematic sectional elevation of the top end cap of thefilter element shown in FIG. 2 illustrating a sixth embodiment of theinlet tube of the filter element.

FIG. 11 is a sectional elevation of the first vane of the filter elementshown in FIG. 2.

FIG. 12 is a sectional elevation of the first vane of the filter elementshown in FIG. 2 according to a second embodiment.

FIG. 13 is a sectional elevation of the first vane of the filter elementshown in FIG. 2 according to a third embodiment.

FIG. 14 is a sectional elevation of the first vane of the filter elementshown in FIG. 2 according to a fourth embodiment.

FIG. 15 is a sectional elevation view of a flow conduit according to theinvention illustrating the calculation of the position of the firstvane.

FIG. 16 is a sectional elevation view of the flow conduit shown in FIG.15 illustrating the calculation of the gap/chord ratio of the vanes.

FIG. 17 is a sectional elevation view of the flow conduit shown in FIG.15 illustrating the calculation of the radius ratio and the aspect ratioof the sub-flow conduits defined by the vanes.

FIG. 18 a is a schematic perspective view of the first piece of a twopiece flow conduit according to the invention.

FIG. 18 b is a schematic perspective view of the second piece of the twopiece flow conduit shown in FIG. 18 a.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings, FIG. 2 shows a filter element which comprisesa cylindrical wall section 2 formed from a filter medium, and top andbottom end caps 4 and 6.

The wall section 2 defines a hollow space 8 within it. The filter mediumof the wall 2 comprises a cylindrical filtration layer 10 and acylindrical anti-reentrainment layer or drainage layer 12 which fitssnugly around the filtration layer on the outside of the filter element.

The top end cap 4 contains a flow conduit 34 which defines a flow path36 for gas which is to be filtered. The flow conduit 34 has a port 80 init for connection to a gauge for measuring the differential pressureacross the filter element. When the filter element is located within ahousing (described in more detail below) the port 80 can be received ina downwardly facing socket in the housing head, forming a seal bycompression of an O-ring between the external surface of the port andthe internal surface of the socket.

The flow conduit 34 has a first opening 162 having a first axis A, and asecond opening 164 having a second axis B. The angle between the axes A,B of the first 162 and second 164 openings is 90°. The flow conduit 34provides a continuous flow path between the two openings, and thereforeprovides a smooth change of direction for gas flowing therethrough whenin use. The flow conduit 34 turns about an axis D which extendsperpendicularly to the axes A, B of the first 162 and second 164 conduitopenings. (As shown in FIG. 2, the axis D about which the flow conduit34 turns extends perpendicularly to the plane along which thecross-section of FIG. 2 of the filter element is taken).

The flow conduit 34 contains first 158, second 159 and third 160 curvedvanes extending perpendicularly across the flow conduit. Each vanecurves around its own axis and the radius of curvature is the same foreach vane. Further, the length of the vanes, measured between theirleading and trailing edges, is the same for each vane. The axes aroundwhich the vanes curve extend perpendicularly to the axes A, B of thefirst and second conduit openings 162, 164. For example, as shown inFIG. 2, the second vane 159 curves around an axis C which extendsparallel to the axis D about which the flow conduit 34 turns. The vaneseach have concave 168 and convex 170 surfaces, wherein the concavesurface of each vane faces the first 162 and second 164 openings of theflow conduit 34. Therefore, the vanes help guide the flow of gas betweenthe first 162 and second 164 openings.

The shape and configuration of the first 158, second 159 and third 160vanes is identical and shown in more detail with reference to the firstvane 158 in FIG. 11. As shown, the first vane 158 has a rounded leadingedge 172 which faces into the direction of the flow of gas 176 when thefilter element is in use, and a trailing edge 174. The thickness of thefirst vane 158 is substantially constant between its leading andtrailing edges. The vane has a straight portion 182 proximal itstrailing edge 174, and a curved portion 186 extending between itsleading edge and the straight portion. The length of the straightportion 182 is 5% of the total length of the vane 158 between itsleading 172 and trailing 174 edges. The angle of incidence of the vaneto the flow of gas when in use (i.e. the angle 180 between a straightline 176 projecting parallel to the direction of the flow of gasimmediately upstream of the vane 158 and a straight line 186 projectingtangentially from the convex surface 170 of the vane at its leading edge172) is 4°.

The first 158, second 159 and third 160 vanes need not have the shapeand configuration of the vane shown in FIG. 11. For example, the vanescan have the shape and configuration of an elbow shaped vane 188 asshown in FIG. 12. The elbow shaped vane 188 is substantially similar inconfiguration to that shown in FIG. 11 and like parts share likereference numerals. However, the elbow vane 188 has a straight leadingportion 190 proximal its leading edge 172, as well as a straighttrailing portion 192 proximal its trailing edge 174 and a curved portion194 between them.

Further, the first 158, second 159 and third 160 vanes can have theshape and configuration of the crescent shaped vane 196 shown in FIG.13. The crescent shaped vane 196 is substantially similar inconfiguration to that shown in FIG. 11 and like parts share likereference numerals. However, the thickness of the crescent shaped vane196 is not uniform between its leading 172 and trailing edges 174.Instead, the thickness of the crescent shaped vane 196 continuouslyincreases as you travel from its leading edge 172 towards the midpointbetween the leading and trailing 174 edges, and continuously decreasesas you travel from the midpoint to the trailing edge.

Further still, the first 158, second 159 and third 160 vanes can havethe shape and configuration of the curved-aerofoil shaped vane 198 shownin FIG. 14. The curved-aerofoil shaped vane 198 is similar inconfiguration to that shown in FIG. 11 and like parts share likereference numerals. However, the thickness of the curved-aerofoil shapedvane 196 is not uniform between its leading 172 and trailing 174 edgesand is generally thicker towards its leading edge than towards itstrailing edge.

Referring now back to FIG. 2, the gap/chord ratio of the first 158,second 159 and third 160 vanes shown in FIG. 2, is 0.45.

The first 158, second 159 and third 160 vanes are arranged such thatradius ratio of all of the sub-flow conduits (described in more detailbelow in relation to FIG. 17) is at least 1.

The first 158, second 159 and third 160 vanes are arranged such thataspect ratio for all the sub-flow conduits is at least 1.

FIGS. 15 to 17 illustrate how the preferred distance between the concavesurface of the inner most vane and wall of the flow conduit the concavesurface of the vane faces, the gap/chord ratio, and the radius ratio andaspect ratio for a flow conduit with vanes, can be calculated. For sakeof simplicity of illustration, the flow conduits 34 of FIGS. 15 to 17contain only a first vane 232 and a second vane 234. Otherwise, allother parts of the flow conduit 34 of FIG. 15 are the same as that shownin FIG. 2 and share the same reference numerals.

As illustrated in relation to FIG. 15, the preferred positioning of theinner most vane (i.e. the first vane 232) can be calculated so that thedistance 200 between its concave surface 168 and wall of the flowconduit the concave surface of the vane faces, is equal to: R-r cos θ/2,where (R) is the radius of the curvature 202 of the centre line 204 ofthe turn of the flow conduit 34, (r) is the radius of the curvature 206of the first vane 232, and θ is the angle between the axes of the first162 and second 164 conduit openings (i.e. 90° as shown in FIGS. 2 and15).

As illustrated in relation to FIG. 16, the gap/chord ratio is calculatedas the ratio of the distance 208 between the midpoints of the first 232and second 234 vanes, and the chords 210 of the vanes.

As illustrated in relation to FIG. 17, the first 232 and second 234vanes divide the flow conduit 34 into first, second and third sub-flowconduits having first 212, second 214 and third 216 centre line of turnsrespectively. The radius ratio of the second sub-flow conduit having acentre line of turn 214, is calculated as the ratio of the radius of thecurvature 218 of the centre line of the turn 214 of the second sub-flowconduit to the width 208 of the sub-flow conduit measured between thefirst 232 and second 233 vanes. Still referring to FIG. 17, the aspectratio of the second sub-flow conduit having a centre line of turn 214 iscalculated as the ratio of the depth of the second sub-flow conduit tothe width 208 of the second sub-flow conduit measured between the vanes.As shown in FIG. 4, the depth of a sub-flow conduit is the distance 220between the opposing walls of the flow conduit 34 between which thevanes that define the sub-flow conduit extend.

Referring now to FIG. 2, the flow conduit 34 and the top end cap 4 ofthe filter element are formed as separate pieces. Further, the flowconduit 34 is formed from two pieces.

A two piece flow conduit 34 is shown in FIGS. 18 a and 18 b. For sake ofsimplicity, the flow conduit 34 of FIGS. 18 a and 18 b contains only afirst vane 232 and a second vane 234. Otherwise, all other parts of theflow conduit 34 of FIG. 15 are the same as that shown in FIG. 2 andshare the same reference numerals. With reference to FIGS. 18 a and 18b, the flow conduit 34 comprises first 221 and second 222 matablepieces. The first 221 and second 222 pieces are mirror images of eachother (the plane of symmetry of the pieces extending through both theaxes of the first 162 and second 164 conduit openings) except that thefirst 232 and second 234 vanes extend from, and are part of, the firstpiece 221 of the flow conduit 34. The first 232 and second 234 vaneshave respective tangs 224, 226 at their free ends (i.e. the ends distalto the wall of the first piece 220 from which the vanes extend) that canbe received within corresponding recesses 228, 230 formed in the wall ofthe second piece 222 of the flow conduit 34. Therefore, when the first221 and second 222 pieces are brought together to form the flow conduit34, the tangs 224, 226 are received within the corresponding recesses228, 230 so that the first 232 and second 234 vanes are secured to thesecond piece 222 at their free ends.

Referring back to FIG. 2, the top end cap 4 further comprises an inlettube 106, co-axial with the top end cap 4, which extends from the secondconduit opening 164 of the flow conduit 34 part way into the hollowspace 8 to a downstream open end. In this embodiment, the inlet tube 106and top end cap 4 are one piece. However, it will be appreciated thatthey need not be one piece. The inlet tube 106 defines a flow path 108for gas which is to be filtered and is in fluid connection with the flowpath 36 of the top end cap 4.

The top end cap 4 has annular opening 112 extending around the inlettube 106 proximal to the flow conduit 34. The inlet tube 106 is heldwithin the annular 112 opening by fins 114 that extend between the topend cap 4 and the inlet tube 106.

As best shown in FIG. 3, the inlet tube 106 has a rifling formationdefined on its inner wall, extending from its end proximal to the topend cap 4, part way towards its end distal to the top end cap. Therifling formation is provided by a helically extending ridge 110 on theinner wall of the inlet tube 106.

Referring now to FIG. 4, a schematic perspective view of the top end cap4 of FIG. 2 is shown. For simplicity and to enable illustration of thehelically extending groove 110, the first vane 158 is not shown. Asshown, the inlet tube 106 has first 116, second 118, third 119 andfourth (not shown) openings in its side wall. The openings are in theform of slits that taper uniformly from an open end at the downstreamopen end of the inlet tube 106, to a point towards the upstream open endof the inlet tube. Therefore, the proportion of the side wall of theinlet tube 106 that is open, increases towards the downstream open endof the inlet tube. The slits extend along 50% of the length of the inlettube and in a direction parallel to the axis of the inlet tube. Theslits are positioned equally around the inlet tube 106, i.e. they arespaced apart from each other by about 90E around the inlet tube.

FIGS. 6 to 10 show alternative embodiments of inlet tubes according tothe invention that can be used with the top end cap 4 of FIG. 2. The topend caps 4 shown in FIGS. 6 to 10 are the same as that shown in FIG. 2.However, for simplicity, the first 158, second 159 and third 160 vanes,and also the helically extending ridge 110, are not shown.

FIG. 6 shows an inlet tube 120 according to the invention which issimilar in configuration to that shown in FIG. 2, except that the first,second, third and fourth slits taper to a flat end 122, instead of to apoint.

FIG. 7 shows an inlet tube 124 which is similar in configuration to thatshown in FIG. 2, except that the inlet tube 124 only has one opening126, which is in the form of a helically extending slit. The helicallyextending slit extends from an open end at the downstream open end ofthe inlet tube 124, to a point towards the upstream open end of theinlet tube. As shown, the angle between the plane perpendicular to theaxis of the inlet tube 124 and a straight line projecting substantiallyalong a portion of the slit 126 is constant. However, the width of thehelically extending slit 126, taken perpendicular to the straight lineprojecting substantially along a portion of the slit, decreases towardsthe upstream open end. Therefore, the proportion of the side wall of theinlet tube 124 that is open, increases towards the downstream open endof the inlet tube.

FIG. 8 shows an inlet tube 128 which is similar in configuration to thatshown in FIG. 7 in that the inlet tube 128 only has one opening 130 inthe form of a helically extending slit. However, in contrast to theinlet tube 124 shown in FIG. 7, the width of the slit 130 shown in FIG.8 in constant along its entire length. Further, the angle between theplane perpendicular to the axis of the inlet tube 128 and a straightline projecting substantially along a portion of the slit 130 decreasestowards the downstream open end of the inlet tube. Therefore, theproportion of the side wall of the inlet tube 128 that is open,increases towards the downstream open end of the inlet tube.

FIG. 9 shows an inlet tube 132 which is similar in configuration to thatshown in FIG. 2, except that the side wall has a plurality of openings134 in the form of apertures. The apertures can be identified as whollyfalling within three bands (an upstream band, designated generally by136, a middlestream band 138, and a downstream band 140), extendingaround the inlet tube, each band defining a plane perpendicular to theaxis of the inlet tube. The upstream band 136 is located within thedownstream half of the inlet tube 132. Four apertures (only one of whichcan be seen in FIG. 9) are present in the upstream band 136, eightapertures (only two of which can be seen in FIG. 9) are present in themiddlestream band 138, and twelve apertures (only three of which can beseen in FIG. 9) are present in the downstream band 140. The apertureshave two equal length, straight, parallel sides, and has a convex end ateach end of the aperture that extends between the parallel sides. Thelength of the parallel sides is longer than the transverse distancebetween them. All of the apertures 134 are equal in shape and dimension.

The four apertures in the upstream band 136 are positioned equallyaround the inlet tube 132, i.e. they are spaced apart from each other byabout 90° around the inlet tube. The eight apertures in the middlestreamband 138 are divided into four sets of two apertures. The four sets oftwo apertures are positioned equally around the inlet tube 132, i.e.they are spaced apart from each other by about 90° around the inlettube. The twelve apertures in the downstream band 140 are divided intofour sets of three apertures. The four sets of three apertures arepositioned equally around the inlet tube 132, i.e. they are spaced apartfrom each other by about 90E around the inlet tube. Therefore, theproportion of the side wall of the inlet tube 132 that is open increasestowards the downstream open end of the inlet tube.

FIG. 10 shows an inlet tube 142 which is similar in configuration tothat shown in FIG. 9, except that the apertures 152 are circular inshape, and that four bands (an upstream band, designated generally by144, an upstream middlestream band 146, a downstream middlestream band148, and a downstream band 150) can be identified instead of three. Fourapertures (only one of which can be seen in FIG. 10) are present in theupstream band 144, eight apertures (only two of which can be seen inFIG. 10) are present in the upstream middlestream band 146, twelveapertures (only three of which can be seen in FIG. 10) are present inthe downstream middlestream band 148, and sixteen apertures (only fourof which can be seen in FIG. 10) are present in the downstream band 150.

Referring back to FIG. 2, each of the end caps has a trough 14 formed init. The top part of the filtration layer 10 and the drainage layer 12are retained and sealed in the trough 14 of the top end cap 4, and thebottom part of the filtration layer is retained and sealed in the trough14 of the bottom end cap 6.

The bottom end cap 6 further comprises a flange part 16, spaced apartfrom the surface 20 of the second end cap facing away from the top endcap 4. The flange part 16 extends generally transverse to the axis ofthe filter element. The flange part 16 is located centrally with respectto the bottom end cap 6, and is spaced from it by a co-axial stem 28extending between them. The flange part 16 and second end cap 6 betweenthem define an annular slot 22, in which the drainage layer can bereceived. The slot is tapered inwardly so that the distance between theflange and the surface of the end cap decreases progressively. The slotis therefore generally V-shaped when the end cap is viewed from oneside.

The bottom part of the drainage layer 12 is wrapped over the wall 18 ofthe trough 14 of the bottom end cap 6 and folded under its bottomsurface 20, in the annular slot 22 between the second end cap 6 and theflange part 16. The drainage layer 12 is fastened in the space 22 bymeans of a loop of elastic material 98, such as an elastic band orO-ring, which can be stretched to fit over the flange. The loop 98 issized so that it is pinched between the opposing surfaces of the slotand the drainage layer: the transverse dimension of the material of theloop is slightly greater than the distance between the drainage layerand the opposing surface of the slot when the loop the tension in theloop (resulting from stretching it to fit it over the flange) isrelaxed. This causes the loop and the drainage layer to be compressedslightly, so that the drainage layer is gripped in the annular slot. Thediameter of the flange is smaller than the diameter of the end cap sothat at least a part 30 of the drainage layer 12 folded over the bottomsurface 20 is exposed.

The bottom end cap 6 also has a central upstand portion 100. The centralupstand portion 100 extends from the bottom of the hollow space definedby the bottom end cap 6 towards the first end cap. The central upstandportion 100 has a generally cylindrical base 102, and a generallyconical part 104 extending from the base toward the first end cap 4. Thediameter of the cylindrical base 102 is the same as that of the stem 28.

Referring now to FIG. 1, an assembly according to the present inventionis shown which includes a housing 50 in which the filter element shownin FIG. 2 can be located when in use. However, as shown, an alternativeembodiment of a filter element which is substantially the same as thatshown in FIG. 2, is located within the housing.

The filter element shown in FIG. 1 is substantially the same as thatshown in FIG. 2, except that an extension 236 is provided on theexternal wall of the flow conduit 34 and extends away from the secondconduit opening 164, instead of the port 80 for connection to a gauge.The extension 236 has seating portion 238 having a generally circularcross-section, and a fin 240 having a generally planar configurationwhich extends between the seating portion 238 and the flow conduit 34.The seating portion 238 provides a generally flat surface 244 and a wall256 which extends around the periphery of the surface and away from thesecond conduit opening 164. A differential pressure measuring device(discussed in more details below) can be received by and containedwithin the area defined by the surface 244 and wall 256 of the seatingportion. The extension 236 is configured so that the axis of thecircular seating portion 238 is angled relative to the axis of thesecond conduit opening 164 by 5°. Therefore, as shown in FIG. 1, whenviewed in cross-section, the seating portion 238 appears slantedrelative to the filter element. A groove 242 that is capable ofreceiving an O-ring is formed around the periphery of the wall 256. Avent 246 that extends between the flow path 36 of the flow conduit 34and the surface 244 of the seating portion 238 is formed within theextension 236.

The housing comprises a head 52 and a body 54 which can be connected toone another by means of cooperating screw threads (as is wellestablished) at their interfaces 86, 88. The head and body are formedfrom a metallic material, especially aluminium or an alloy thereof. Theycan be formed by machining, or by techniques such as casting.

The housing body comprises a cylindrical wall 55, an end wall 57 at oneend of the internal wall, and an open end at the opposite end of thecylindrical wall. The housing body defines a space within which thefilter element is coaxially located when in use. Liquid drops whichdrain from the drainage layer are collected in a reservoir 60 in thehousing body. The housing includes a drain outlet 62, for example of thekind which is disclosed in European Patent No. EP-A-81826.

The filter element fits on to the housing body by means ofinter-engaging formations in the form of ribs and grooves. The top endcap 4 has first 90, second 91, third 154 and fourth 156 ribs (forexample as shown in FIGS. 4 and 5) around its perimeter that extend fromthe top end cap towards the bottom end cap, on the exterior of thefilter element. The four ribs are spaced apart about 90° around the topend cap. Further, the first 90 and second 91 ribs are spaced about 180°around the top end cap. The first 90 and second 91 ribs are received inthe housing body by means of correspondingly shaped and positioned first92, second 93, grooves provided in the interior of the housing body atthe open end.

The third and fourth ribs 154, 156 are identical in shape size andconfiguration. The leading edge of the third and fourth ribs 154, 156(which is directed into the gas stream) is rounded and the trailing edgeof the rib is tapered inwardly, towards (optionally to) a sharp edge orpoint. These ribs 154, 156 are approximately aerofoil-shaped when viewedin cross-section (perpendicular to the axis of the assembly). This shapegives minimal resistance to the flow of gas past the ribs. In contrastto the first 90 and second 91 ribs, there are no corresponding groovesfor the third 154 and fourth 156 ribs. Instead, the axial faces of thethird 154 and fourth 156 ribs match the profile of the housing body sothat they rest against the body when the filter element has been fittedwithin the housing body.

The first and second ribs 91, 90 are shaped differently to that of thirdand fourth ribs 154, 156. The leading edge of the first and second ribs90, 91 are rounded, and the trailing end is flared outward from therounded leading edge. The first and second ribs 90, 91 have anapproximately tapered “V” shape when viewed in cross-section(perpendicular to the axis of the assembly) with a flat top surfaceextending between the ends of the “V”.

As can be seen in FIG. 1, the second rib 91 is located under theinternal cylindrical wall 72 of the head 52 of the housing 50 (discussedin more detail below). Therefore, when the filter assembly is in use,the tapered sides of the second rib 91 aids the flow gas around theinternal cylindrical wall 72, and towards the outlet port 58. The firstrib 90 is located below the output port 58. Therefore, in order tominimise the direction of gas away from the output port, the width ofthe first rib 90 at its widest point is smaller than that of the secondrib 91. Further, due to the first rib 90 and its corresponding firstgroove 92 being narrower than the second rib 91 and its correspondingsecond groove 93, the filter element can only be inserted into the bodyin one orientation. This can ensure that when the housing is assembled,the inlet port 56 of the housing head 52 (described in more detailbelow) is aligned with the flow conduit 34 of the top end cap 4, ratherthan incorrectly aligned with the outlet port 58 of the housing head.This is especially true when the housing head and body are configured sothat they can only fit together in one orientation. This can be achievedby providing a single start screw thread at the interfaces 86, 88 of thehousing head and body.

The filter element is assembled in the housing body by locating thefirst and second ribs 90, 91 with the first and second grooves 92, 93 ofthe housing body 54, and then sliding the ribs into the grooves untilthey sit on the bottom of the grooves. Once the ribs have been fullyreceived by the grooves, the filter element is securely suspended withinthe housing body. Therefore, as will be appreciated, the axial positionof the filter element within the housing body can be controlled by theshape and size of the ribs and grooves. Further, rotation of the filterelement relative to the housing body is inhibited by the interlocking ofthe ribs with the grooves.

Once the filter element is appropriately assembled in the housing body,an annular space 64 is defined between the filter element and thehousing. The filter element can be removed from the housing body 54, bypulling the filter element away from the housing body along its axis.

The housing body 54 has two a plurality of fins 96 extending along thecylindrical wall 55, parallel to the axis of the housing body. Thenumber of fins 96 provided on the cylindrical wall 55 of the housingbody 54 can depend on the size of the housing body. For example, ingeneral, the larger the housing body, the greater the number of fins 96provided. Typically, the minimum number of fins 96 provided will be two.In the embodiment shown, the housing body has six fins 96, however, onlytwo are shown. The fins extend from the end wall 57 of the housing bodytowards the open end, and are spaced uniformly around the cylindricalwall 55 of the housing body 54. When the filter element is assembled inthe housing body so that the ribs have been fully received within thegrooves, the part of the drainage layer 12 which extends over the bottomend cap 6 is compressed between the bottom end cap and the edge of thefins 96. Accordingly, the transverse position of the filter elementwithin the housing body can be controlled by the shape and size of thefins. The cross-section of the edge of the fins which contacts thedrainage layer, taken perpendicular to the length of the ridge, is arounded convex shape. As a result of the local compression of thedrainage layer between the fins and the second end cap, liquid collectedin the drainage layer is encouraged to drain from it, along each fin.

The housing head includes an inlet port 56 which communicates with theflow conduit 34 on the top end cap 4 through a transversely extendingprimary chamber 68 within the housing head. The primary chamber 68 isdefined by an internal cylindrical wall 72 extending transversely withinthe housing head, and an internal end wall 70 opposite the inlet port56. The internal cylindrical side wall 72 and end wall 70 are integralto the housing head. A first circular aperture 74, coaxial with thehousing head, is defined within the part of side wall of the inletconduit that is proximal to the filter element when assembled. A recess248, that extends away from the filter element when the assembly isassembled, and that is coaxial with the housing head, is formed withinthe primary chamber 68. Further, a vent 250 that extends between theprimary chamber 68 and the area surrounding it is formed in the end wall70.

The housing head 52 is secured to the body 54 (once the filter elementhas been located in the housing body) by locating the flow conduit 34 ofthe top end cap 4 in the primary chamber 68 of the housing head throughthe circular aperture 74. The flow conduit 34 has an O-ring 66 on itsexternal surface which is received by the aperture 74, in which it iscompressed to form a fluid tight seal. The seating portion 238 also hasan O-ring 252 which is compressed between the groove 242 of the seatingportion and the side wall of the recess 248 within primary chamber 68,to form a fluid tight seal, thereby defining an auxiliary chamber 254within the primary chamber.

The housing head 52 and body 54 are secured by rotating one relative tothe other so that their cooperating screw threads at their interfaces86, 88, are tightened to interlock with each other. Once the head isproperly secured to the body 54 the vent 250 in the end wall 70 of thehousing head 52 extends between the auxiliary chamber 254 and the areawithin the housing head surrounding the primary chamber 68.

A device 258 for measuring differential pressure is contained within thearea defined by the surface 244 and the wall 256 of the seating portion238. Suitable differential pressure measuring devices are known inexisting products, for example in products sold by Domnick HunterLimited under the trade mark OIL-X. The device 258 can be secured to theseating portion 238 by frictional forces between it and the wall 256 ofthe seating portion 238; i.e. the device is a secured to the seatingportion 238 by a “press-fit” connection. However, other techniques canbe used to secure the pressure measuring device, for example a threadedengagement or an adhesive. An O-ring 260 is provided on the pressuremeasuring device 258 which is compressed between the device and the wall256 to provide a fluid tight seal between the vent 246 within theextension 236 and the axiliary chamber 254. As the vent 246 within theextension 236 is in fluid communication with gas upstream of the filterelement and the auxiliary chamber 254 is in fluid communication with gasdownstream of the filter element via the vent 250 in the end wall 70,the pressure measuring device is capable of measuring the pressure dropacross the filter element.

The filter assembly can be disassembled by rotating the housing body 54relative to the head 52 so that their cooperating screw threads areloosened. Any rotational force that is imparted on the top end cap 4 ofthe filter element by frictional forces between the O-rings 66, 252 ofthe end cap 4 and the housing head is negated by the opposite rotationaldrive that is provided by the first and second ribs 90, 91 actingagainst the first and second grooves 92, 93 in the housing body in whichthe ribs are received. Therefore, as the housing body 54 is rotatedrelative to the housing head 52, the filter element will tend to residein the housing rather than be drawn away from the body with the head,and hence when the housing head 52 is removed from the housing body 54,the filter element will remain located within the body, rather thanbeing removed from the body with the head.

The housing head includes an outlet port 58 through which gas which haspassed through the wall 2 of the filter element can be supplied to adownstream application. The outlet port communicates with the annularspace 64 between the wall of the filter element and the internal wall ofthe housing.

In use, a gas that is to be filtered enters the filter assembly throughthe inlet port 56 in the housing head and is directed to the hollowspace 8 in the filter element by means of the primary chamber 68 in thehousing head and the flow paths, 36 and 109 in the flow conduit of thefilter element, and the inlet tube, respectively. A helical flow isimparted in the gas stream entering the hollow space 8 by the helicallyextending ridge 110, as the gas stream passes through the inlet tube.The supply of gas entering the hollow space 8 is graded due to thegradual increase in the proportion of the inlet tube that is opentowards its downstream end.

From the hollow space 8, the gas flows generally radially outwardlythrough the filter medium of the wall 2. Any liquid in the gas streamwill be coalesced by the filtration layer 10 and any coalesced liquidwill be carried to the drainage layer 12 by the flow of gas, where theliquid will be retained. The liquid will tend to drain to the bottom ofthe drainage layer 12, where it can tend to accumulate in the part ofthe drainage layer 12 folded under the bottom surface 12, therebyforming a wet band. When that part of the drainage layer 12 becomessufficiently saturated, the liquid will begin to drain from any exposedparts of that part of the drainage layer, generally in the form ofdrops. The compression of the drainage layer 12 by the fins 96 will tendto encourage the drainage of liquid from the drainage layer along thefins.

Filtered gas exiting the filter element enters the annular space 64between the filter element and the housing. Filtered gas is thendischarged from the filter assembly through the outlet port 58 in thehousing head 52, which is in fluid communication with the annular space64. Gas flowing from the annular space 64 to the outlet port 58 isdirected around the internal cylindrical wall 72 of the housing head 52,and toward the outlet port, by the tapered rib 91 that is locateddirectly underneath the cylindrical side wall.

1. A filter element for removing material that is entrained in a gasstream, the filter element comprising: a wall of a filtration mediumwhich defines a hollow space, for a gas stream to flow from the spacethrough the wall, and an end cap having an inlet tube for a gas streamto be supplied to the space, the tube having at least one helicallyextending rifle formation in its internal wall by which a helical flowis imparted to the gas stream when it leaves the tube.
 2. A filterelement as claimed in claim 1, in which the rifle formation is providedby at least one ridge which is provided on the internal wall of thetube.
 3. A filter element as claimed in claim 2, in which the rifleformation comprises a plurality of ridges which are arranged so that thedistance between adjacent ridges is approximately constant over thelength of the ridges.
 4. A filter element as claimed in claim 2, inwhich the ridge is tapered towards a point at least one of its ends. 5.A filter element as claimed in claim 1, in which the rifle formation isprovided by at least one groove which is provided on the internal wallof the tube.
 6. A filter element as claimed in claim 5, in which thegroove is defined by two ridges which are provided on the internal wallof the tube.
 7. A filter element as claimed in claim 1, in which thehelix angle defined by the formation is at least about 5°.
 8. A filterelement as claimed in claim 1, in which the helix angle defined by theformation is not more than about 75°.
 9. A filter element as claimed inclaim 1, in which the tube extends into the hollow space that is definedby the wall of the filtration medium.
 10. A filter element as claimed inclaim 9, in which the tube has at least one opening in its side wallthrough which a gas stream within the tube can leave the tube, otherthan at the end of the tube that is within the hollow space.
 11. Afilter element as claimed in claim 10, in which the opening is in theform of a slit.
 12. A filter element as claimed in claim 1, in which therifle formation extends through at least 360° between its ends.
 13. Afilter element as claimed in claim 1, in which the ratio of the depth ofthe rifle formation to the transverse dimension of the tube is at leastabout 0.02.
 14. A filter element as claimed in claim 1, in which thedepth of the rifle formation is at least about 0.5 mm.
 15. A filterassembly which comprises a filter element as claimed in any one of thepreceding claims, and a housing in which the filter element can befitted when in use.
 16. A filter assembly as claimed in claim 15, inwhich the housing comprises a head part and a body part, which can beseparated to provide access to the interior of the housing.